In order to selectively elevate the sensitivity of a light-sensitive silver halide emulsion to a light falling within a particular wavelength range, employed is a color-sensitizing technique of adding a color-sensitizing dye to the silver halide emulsion.
It is important that color-sensitizing dyes not only have a color-sensitizing activity as their intrinsic function but also have no photographically-harmful effect on photographic materials. For instance, photographically-harmful effects of color-sensitizing dyes are such that the dyes remain in processed photographic materials to cause color stains and that they cause lowering of sensitivity of photographic materials or increasing of fog in them during storing photographic materials. Conventional improvements in color-sensitizing dyes have eliminated many of these harmful effects, but further improvements in them are still desired. On the other hand, since the adsorbability of conventional color-sensitizing dyes onto fine silver halide crystals varies with the lapse of time while silver halide emulsions containing them are coated on a support to produce photographic materials, it is often difficult to produce photographic materials having constant photographic properties. Therefore, it is desired to improve color-sensitizing dyes so as to evade the difficulty.